The invention relates to an electric lamp provided with a light source in a light-transmitting lamp vessel which is closed in a vacuumtight manner, which light source has an envelope of light-transmitting, UV-absorbing quartz glass which contains aluminium in oxidic form and a metal in oxidic form chosen from a group to which cerium and titanium belong.
A high-pressure discharge lamp of this kind is known from U.S. Pat. No. 3,531,677.
The known lamp has a quartz glass lamp vessel which has a layer of doped quartz glass at its outer surface. The doping in this case consists of a coloured, possibly UV-absorbing oxide or of cerium oxide or titanium oxide as a colourless, UV-absorbing substance. A disadvantage of the known lamp is that the lamp is impervious to only a portion of the UV spectrum and, dependent on the doping, also transmits only a portion of the light generated by the light source.
NL 77 14 305 discloses a high-pressure discharge lamp whose quartz glass lamp vessel consists of UV-absorbing doped quartz glass in chosen locations, for example, quartz glass with 0.02 mole % of each of the element europium, cerium, and titanium. The doped quartz glass here is present only in those locations of the lamp vessel where the latter, depending on the position which it occupies during operation, would have a comparatively low temperature without a dopant. Owing to the doping, UV is absorbed and the temperature in situ increases, which benefits the luminous efficacy of the lamp. The doped quartz glass, however, has a yellow colour so that it does not transmit all the generated light. Moreover, it does not absorb all UV radiation.
U.S. Pat. No. 4,361,779 discloses an electric lamp, a halogen incandescent lamp and a high-pressure discharge lamp, whose quartz glass lamp vessel has a doping of alkali oxide, alkaline earth oxide, rare earth oxide such as praseodymium oxide, possibly with aluminium oxide or europium oxide. The lamp vessel is strongly yellow-coloured and as a result is suitable for use in lamps, for example halogen lamps, which are to emit yellow light. The glass is closed to only a portion of the UV spectrum.
An electric incandescent lamp and a high-pressure discharge lamp with a doped quartz glass lamp vessel or with an outer envelope of such a glass are known from U.S. Pat. No. 5,196,759. The doping here consists of titanium oxide and cerium oxide and has the object of absorbing a portion of the UV radiation. Among the wavelengths which are not absorbed, however, is comparatively long-wave UV radiation.
A UV absorbing quartz glass and electric lamps having an envelope of that quartz glass are described in the European patent application of older date EP-A 0 574 158. The quartz glass contains at least 96% by weight of SiO.sub.2 and europium oxide, titanium oxide and cerium oxide.
From this patent application it is apparent (FIG. 1) that the addition of europium oxide diminishes the UV-transmission of quartz glass doped with cerium oxide and titanium oxide, but also its transmission in the visible range of the spectrum, because it colours the glass yellow.